1. Field of the Invention
A transmission line driver/receiver is disclosed. More particularly, a bidirectional transmission line driver/receiver having an active transmission line terminator is disclosed.
2. Description of Related Art
As the operating speed of computer processors continues to increase, additional demands are placed on devices that must send and receive logic signals.
In measuring high frequency signals, the measurement sample rates are, of course, higher than in measuring lower frequency signals. Measuring the precise magnitude of a signal at high sample rates is more difficult than at lower rates because transient conditions, which may not have any appreciable effect within the relatively long time period of a lower frequency sample rate, may have a significant effect in the shorter period of a high frequency sample rate.
One high frequency effect of particular concern in the present invention is that of signal reflection, which occurs when the impedance of a communication path does not match the impedance of a terminating load on the receiving end of the path. (The communication path between a sending and a receiving device includes a path such as a buss between devices on a single circuit board or a cable between devices on different boards. In any case, because the present invention concerns relatively high frequency signals which require a consideration of transmission line effects, the communication path may be referred to herein as a transmission line.) Ideally, a terminating load will sink a transmitted signal immediately upon the signal arriving at the load so that the magnitude of the signal may be sampled immediately upon its arrival at the load and a precise value of the signal magnitude may be determined from the sample. However, if the terminating load does not match the transmission line impedance, then the load will sink only a portion of the signal upon the signal's initial arrival. The remaining portion of the signal will be reflected back onto the transmission line. Typically, depending upon a number of factors, at least a part of this reflected portion of the signal will ultimately be reflected from other loads on the transmission line back to the terminating load, and this reflection back and forth among loads on the line will repeat, until eventually more and more of the signal will sink through the terminating load.
The effect of this signal reflection is, at best, to delay the time at which the signal-may be accurately sampled until such time as the signal has gone through perhaps many cycles of reflection on the transmission line. At worst, in the case where a significant part of the reflected portion is dissipated in the transmission line itself, or sinks in other loads on the transmission line, the effect of this reflection is to greatly reduce the precision with which the magnitude of the signal can be measured, perhaps to the point where the logic value of a signal cannot be accurately determined at all. Therefore, it is advantageous to eliminate, or at least greatly reduce, signal reflection by matching, insofar as possible, the impedance of a terminating load and that of a transmission line.
It is well known to send multivalued logic signals from a sending device to a receiving device in order to facilitate simultaneous bidirectional transmission. For a multi-valued logic signal, a receiver must be more discriminating than a receiver that merely detects an on condition and an off condition. That is, a receiver for multi-value logic signals must measure the magnitude of a received signal with a relatively higher degree of precision than a receiver that must merely detect the presence or absence of a signal. Thus, there is even greater incentive for impedance matching for high frequency signals which are multi-valued.
While it is relatively straightforward and economical to manufacture a transmission line between devices with tight control of the line impedance, it is not as simple to economically manufacture a fixed terminating load within tight impedance limits. Therefore, an object of the present invention is to actively adjust termination impedance and logic signal current during operation to eliminate or greatly reduce signal reflection using components which may be manufactured economically.
Impedance matching using active devices could be achieved by providing, in addition to the transmission signal, a first signal over a first reference line from a sender to a receiver informing the receiver of the amplitude for a logic signal, and a second signal over a second reference line from a receiver to a sender informing the sender of the impedance of a transmission line terminator in the receiver. However, particularly where a number of senders and receivers are interconnected on the transmission line, this arrangement is disadvantageous because of the number of reference lines required. Therefore, another object of the invention is to minimize signal reflection with a single reference line between devices on the transmission line.